The protozoan Toxoplasma gondii can cause severe opportunistic disease (toxoplasmosis) in immune-deficient individuals, yet up to one third of the human population is benignly infected. The long-term goal of this project is to determine how Toxoplasma efficiently establishes infection in the midst of the host's highly capable innate immune system. An understanding of the basic mechanisms used by microbial pathogens to subvert host immune defenses is necessary to rationally develop novel strategies for preventing infection. Our central hypothesis is that Toxoplasma uses natural serine proteases (serpins) to control innate immune defenses including inflammation and complement fixation via the alternative pathway. This is based on key roles for protease inhibitors in the regulation of inflammation and complement. Other studies have also established that several viruses and helminth parasites deploy serpins to subvert innate immunity. In line with the goals of the RO3 funding mechanism, this grant is designed to generate preliminary data in support of subsequent full scale testing of hypotheses. The specific aims are: (1) to create parasite strains deficient in expression of the serpins TgPI-1 and TgPI-2; (2) to test whether TgPl's possess anti-inflammatory activity; and (3) to test whether TgPl's block alternative complement fixation. The impact of this research will be a better understanding of the mechanisms permitting parasite survival during infection, with the added benefit of potentially leading to progressive approaches for boosting innate immunity and treating inflammatory diseases.